Minecraft PlotSquared plugin help
Solution 1:
The problem seems to be you do not have a permissions plugin.
Plugins like multiverse and plotsquared are made to work with a permissions handler. The way it sounds, every player in the specified world does not have permissions restricting them from building anywhere and the plotsquared plugin alone does not handle what players have access to in terms of building without a permissions plugin.
The most common plugin for this is here: http://www.curse.com/bukkit-plugins/minecraft/permissionsex